The present invention relates to the field of plasma spray guns and particularly to a plasma spray gun nozzle with a thin annular coolant passage which increases the nozzle life over that previously achieved with prior art designs.
In typical plasma flame spraying systems, an electrical arc is created between a water cooled nozzle (anode) and a centrally located cathode. An inert gas passes through the electrical arc and is excited thereby to temperatures of up to 30,000.degree. F. The plasma of at least partially ionized gas issuing from the nozzle resembles an open oxy-acetylene flame. A typical plasma flame spray gun is described in U.S. Pat. No. 3,145,287.
The electrical arc of such plasma spray guns, being as intense as it is, causes nozzle deterioration and ultimate failure. One cause for such deterioration is the fact that the arc itself strikes the nozzle/anode at a point, thereby causing instantaneous melting and vaporizing of the nozzle surface. Deterioration is also caused by overheating the nozzle to the melting point so that part of the nozzle material flows to another location which may eventually cause the nozzle to become plugged.
There are varying degrees and rates associated with each cause for nozzle deterioration. Experience has shown that wall erosion, ultimately causing the coolant to burst through the nozzle wall, is another cause of nozzle failure. When the jacket bursts, coolant water is released into the arc region, resulting in a locally intense electric arc, causing parts to melt. Once a meltdown has occurred, gun repair can be very costly. The nozzle deterioration and failure problem is particularly severe at high power levels.
In seeking to overcome this problem, plasma flame spray guns have been designed with easily changed water cooled nozzles. During operation, water coolant is forced through passages in the nozzle to cool the nozzle walls. Even so, gradual, or sometimes rapid, deterioration occurs and, as a precaution against failure, the nozzles are usually replaced after a given number of hours of service. This practice of replacing the nozzle periodically, however, is quite costly because the interchangable nozzles are fairly expensive and many nozzles with considerable life remaining are thereby discarded.
Many factors are involved in determining the rate of deterioration and ultimate failure of a plasma spray gun nozzle. For the most part, nozzle operating conditions and geometry, gas type and flow rate, coolant flow rate and velocity influence the nozzle life, as well as does the nozzle cooling.
Some installations of plasma spraying equipment have included deionizers in the coolant system which, as indicated by recent studies, has enhanced the life of the nozzle. The reason for the nozzle life enhancement apparently arises from a reduction of scale formation within the coolant passages of the nozzle. However, under the more severe operating conditions, e.g. high power level, use of a deionizer alone is not sufficient to significantly improve nozzle life.
The prior art generally recognizes that cooling the nozzle wall is necessary and has the above-noted effect on nozzle life. The prior art, however, does not recognize the optimum design for nozzles and cooling passages in plasma flame spray guns, thus leaving the designer to endless experimentation in attempting to determine the optimum design for maximum nozzle life.
Therefore, it is the primary objective of the present invention to provide a plasma flame spray system designed to maximize nozzle life.
It is a further objective of the present invention to provide a nozzle for a plasma flame spray gun which is designed to maximize the operational life thereof.
It is still a further objective of the present invention to provide a nozzle for a plasma flame spray gun with a coolant passage therein designed to improve heat removal from the nozzle wall.
It is yet a further objective of the present invention to provide a nozzle for a plasma flame spray gun having a wall thickness which maximizes the nozzle life as defined by the equation EQU Life=(W.sub.start -W.sub.min)/R
where W.sub.start is the initial wall thickness, W.sub.min is the wall thickness at failure and R is the erosion rate in depth per unit time.
Another objective of the present invention is to provide a nozzle for a plasma flame spray gun having a wall thickness and coolant passage therein designed to minimize melting and flow of nozzle material, and thereby to reduce failure by plugging of the nozzle.